Generally, permanent-magnet-type rotating electrical machines are classified into two types. One is a surface-permanent-magnet-type rotating electrical machine having permanent magnets adhered to an outer circumferential face of a rotor core and the other is an internal-permanent-magnet-type rotating electrical machine having permanent magnets embedded in a rotor core. For a variable-speed driving motor, the internal-permanent-magnet-type rotating electrical machine is appropriate.
With reference to FIG. 20, a configuration of a conventional internal-permanent-magnet-type rotating electrical machine will be explained. Along an outer circumference of a rotor core 2 of a rotor 1, rectangular hollows are arranged at regular intervals, the number of the rectangular hollows being equal to the number of poles. In FIG. 20, the rotor 1 has four poles, and therefore, four hollows are arranged to receive permanent magnets 4, respectively. Each permanent magnet 4 is magnetized in a radial direction of the rotor 1, i.e., in a direction orthogonal to a side (long side in FIG. 20) of the rectangular section of the permanent magnet 4 that faces an air gap. The permanent magnet 4 is usually an NdFeB permanent magnet having high coercive force so that it is not demagnetized with a load current. The rotor core 2 is formed by laminating electromagnetic sheets through which the hollows are punched. The rotor 1 is incorporated in a stator 20. The stator 20 has an armature coil 21 that is received in a slot formed inside a stator core 22. An inner circumferential face of the stator 20 and an outer circumferential face of the rotor 1 face each other with the air gap 23 interposing between them.
Known examples of such a permanent-magnet-type rotating electrical machine are described in “Design and Control of Internal Magnet Synchronous Motor,” Takeda Yoji, et al., a document of Ohm-sha Publishing (Non-Patent Document 1) and Japanese Unexamined Patent Application Publication No. H07-336919 (Patent Document 1). An example of a high-output rotating electrical machine having an excellent variable-speed characteristic is a permanent-magnet-type reluctance motor. Examples thereof are described in Japanese Unexamined Patent Application Publication No. H11-27913 (Patent Document 2) and Japanese Unexamined Patent Application Publication No. H11-136912 (Patent Document 3). Further, there is an internal permanent motor employing AlNiCo magnets whose magnetic force is changed. Examples thereof are described in U.S. Pat. No. 6,800,977 (Patent Document 4) and Weschta, “Schachung des Erregerfelds bei einer dauermagneterregten Synchronmaschine”, ETZ Archiv Vol. 7, No. 3, pp. 79-84 (1985) (Non-Patent Document 2).
The rotating electrical machine of the Non-Patent Document 2 is a permanent magnet motor employing AlNiCo magnets, flux amount of the AlNiCo magnets being changed. This related art may demagnetize the AlNiCo magnets but it hardly magnetizes the AlNiCo magnets to an original magnetized state. The rotating electrical machine mentioned in the Patent Document 4 is a flux-concentration-type internal permanent magnet motor employing AlNiCo permanent magnets. This rotating electrical machine is a modification of the rotating electrical machine described in the Non-Patent Document 2, and like the rotating electrical machine of the Non-Patent Document 2, applies a magnetic field to change the flux amount of the AlNiCo magnets. The rotating electrical machine of the Patent Document 4 is a simple AlNiCo magnet motor, and therefore, provides insufficient output. When torque is produced in the rotating electrical machine of the Non-Patent Document 2 or of the Patent Document 4, the AlNiCo magnets are demagnetized by a load current, to decrease the torque. To generate sufficient torque with the AlNiCo magnets whose energy product is small, the AlNiCo magnets must be thick in a magnetizing direction. To magnetize such thick AlNiCo magnets, a very large current must be passed. Namely, the permanent magnets are hardly magnetized and the flux amount thereof becomes unchangeable.
In the permanent-magnet-type rotating electrical machine, the permanent magnets always generate constant linkage flux to increase a voltage induced by the permanent magnets in proportion to a rotation speed. Accordingly, when variable-speed operation is carried out from low speed to high speed, the voltage (counter electromotive voltage) induced by the permanent magnets becomes very high at high rotation speed. The voltage induced by the permanent magnets is applied to electronic parts of an inverter, and if the applied voltage exceeds a withstand voltage of the electronic parts, the parts will cause insulation breakage. It is necessary, therefore, to design the machine so that the flux amount of the permanent magnets is suppressed below the withstand voltage. Such a design, however, lowers the output and efficiency of the permanent-magnet-type rotating electrical machine in a low-speed zone.
If the variable-speed operation is carried out in such a way as to provide nearly a constant output from low speed to high speed, the voltage of the rotating electrical machine will reach an upper limit of a power source voltage in a high rotation speed zone, not to pass a current necessary for output because the linkage flux of the permanent magnets is constant. This greatly drops the output in the high rotation speed zone, so that the variable-speed operation will not be carried out in a wide range up to high rotation speed.
Recent techniques of expanding a variable-speed range employ flux-weakening control such as one described in the Non-Patent Document 1. A total linkage flux amount of an armature coil is the sum of flux by a d-axis current and flux by permanent magnets. The flux-weakening control generates flux with a negative d-axis current to reduce the total linkage flux amount. The flux-weakening control makes a permanent magnet of high coercive force operate in a reversible range on a magnetic characteristic curve (B-H characteristic curve). For this, the permanent magnet must be an NdFeB magnet of high coercive force so that it may not irreversibly demagnetized with a demagnetizing field produced by the flux-weakening control.
In the flux-weakening control, flux produced by a negative d-axis current reduces linkage flux and a reduced portion of the linkage flux produces a voltage margin for an upper voltage limit. This makes it possible to increase a current for a torque component, thereby increasing an output in a high-speed zone. In addition, the voltage margin makes it possible to increase a rotation speed, thereby expanding a variable-speed operating range.
Always passing a negative d-axis current that contributes nothing to an output, however, increases a copper loss to deteriorate efficiency. In addition, a demagnetizing field produced by the negative d-axis current generates harmonic flux that causes a voltage increase. Such a voltage increase limits the voltage reduction achieved by the flux-weakening control. These factors make it difficult for the flux-weakening control to conduct the variable-speed operation for the internal-permanent-magnet-type rotating electrical machine at speeds over three times a base speed. In addition, the harmonic flux increases an iron loss to drastically reduce efficiency in middle- and high-speed zones. Further, the harmonic flux generates an electromagnetic force that produces vibration.
When the internal-permanent magnet motor is employed as a drive motor of a hybrid car, the motor rotates together with an engine when only the engine is used to drive the hybrid car. In this case, voltage induced by permanent magnets of the motor increases at middle and high rotation speeds. To suppress the induced voltage below a power source voltage, the flux-weakening control continuously passes the negative d-axis current. The motor in this state produces only a loss to deteriorate an overall operating efficiency.
On the other hand, when the internal-permanent magnet motor is employed as a drive motor of an electric train, the electric train sometimes carries out a coasting operation. Then, like the above-mentioned example, the flux-weakening control is carried out to continuously pass the negative d-axis current to suppress voltage induced by permanent magnets below a power source voltage. The motor in this state only produces a loss to deteriorate an overall operating efficiency.
A technique to solve these problems is disclosed in Japanese Unexamined Patent Application Publication No. 2006-280195 (Patent Document 5). The technique described in the Patent Document 5 relates to a permanent-magnet-type rotating electrical machine capable of conducting variable-speed operation in a wide range from low speed to high speed and improving efficiency and reliability. This machine includes a stator provided with a stator coil and a rotor having low-coercive-force permanent magnets whose coercive force is of such a level that a magnetic field created by a current of the stator coil may irreversibly change the flux density of the magnets and high-coercive-force permanent magnets whose coercive force is equal to or larger than twice that of the low-coercive-force permanent magnets. In a high rotation speed zone in which the voltage of the machine exceeds a maximum power source voltage, a current is passed to create a magnetic field that magnetizes the low-coercive-force permanent magnets in such a way as to reduce total linkage flux produced by the low-coercive-force and high-coercive-force permanent magnets, thereby adjusting a total linkage flux amount.
A permanent-magnet-type rotating electrical machine described in Japanese Unexamined Patent Application Publication No. H07-336980 (Patent Document 6) is a brushless DC motor. This brushless DC motor employs a rotor core having a small-coercive-force first magnet part and a large-coercive-force second magnet part. Flux amount of a magnetic pole of the rotor core is reduced by applying power to an armature coil, to invert only a magnetizing direction of the small-coercive-force first magnet part. This realizes a flux reduction without always passing an opposite field current to the armature coil during demagnetization.
In the case of this conventional brushless DC motor, the kind of magnets adopted for the first and second magnet parts is unclear and the magnetic characteristic thereof is unidentifiable because FIG. 7 indicates no values. However, the shapes of magnetic characteristic graphs thereof allow an estimation that the small-coercive-force first magnet part seems to be made of ferrite-based permanent magnets and the large-coercive-force second magnet part of NdFeB permanent magnets. Even if the two kinds of permanent magnets having large and small coercive forces as illustrated in the magnetic characteristic curves of FIG. 7 are employed, the ferrite-based permanent magnets have small coercive force, and therefore, are easily demagnetized to raise the problem of demagnetization even with a magnetic field produced by a q-axis torque current, the necessity of a large current for demagnetization, the problem of insufficient torque, and the like.
On the other hand, if a permanent magnet having a coercive force of 300 kA/m or over is employed as the permanent magnet to be demagnetized, a large current must be passed to demagnetize the same, to raise the problems of enlarging a power source and saturating parts around the permanent magnet due to a magnetic field created by the demagnetizing current, thereby preventing the demagnetization of the permanent magnet. If a remnant flux density of the permanent magnet is smaller than 0.6 T like a ferrite-based permanent magnet (0.45 T in the case of a ferrite magnet), the width of change in flux amount is small to raise the problem of narrowing an output variable width.    Patent Document 1: Japanese Unexamined Patent Application Publication No. H07-336919    Patent Document 2: Japanese Unexamined Patent Application Publication No. H11-27913    Patent Document 3: Japanese Unexamined Patent Application Publication No. H11-136912    Patent Document 4: U.S. Pat. No. 6,800,977    Patent Document 5: Japanese Unexamined Patent Application Publication No. 2006-280195    Patent Document 6: Japanese Unexamined Patent Application Publication No. H07-336980    Non-Patent Document 1: “Design and Control of Internal Magnet Synchronous Motor,” Takeda Yoji, et al., Ohm-sha Publishing    Non-Patent Document 2: Weschta, “Schachung des Erregerfelds bei einer dauermagneterregten Synchronmaschine”, ETZ Archiv Vol. 7, No. 3, pp. 79-84 (1985)